DIVISION OF THE LIGHT STRIKING A BODY 155 



or self-luminous when we see it below this temperature? When we see 

 it above this temperature? 



2. What reasons have we for believing that the moon is not self- 

 luminous? 



3. Classify the following substances as transparent, translucent, or 

 opaque: ice, snow, alcohol, milk, iron, the material of our finger nails, 

 skin, tissue paper, carbon dioxide, a solution of blue vitriol. 



4. I see a carpenter, down the street, driving nails; but the sound of 

 each blow comes to me just as I see the hammer raised for the next blow. 

 Why? 



5. A flash of lightning is seen 10 seconds before the thunder is heard. 

 How far away is the thunder cloud? 



6. Sirius, the brightest of the fixed stars, is about 8 light-years, 

 distant (c/ 11). How many miles is this, if we call the speed of light 

 200,000 miles a second? 



7. What are the positions of the sun, the moon, and the earth when 

 we have " new moon "? When we have " full moon "? At which of 

 these times may we have an eclipse of the moon? At which an eclipse 

 of the sun? 



8. Why is the shadow cast by a light of small diameter, such as an 

 electric bulb, so distinct? 



9. I sit 5 feet from a lamp, while my brother is 1 foot from the 

 lamp. How much more light does he get than I? 



10. In a photometer (Fig. 14,5) a candle at a distance of 1 foot casts 

 a shadow just as dark as one cast by a lamp 6 feet away. What is the 

 lamp's candle power? 



173. Division of the Light Striking a Body. When 

 light meets a body, several things may happen: 



(1) The light may be thrown back reflected in 

 a regular way and in certain definite directions. 



(2) It may be reflected irregularly dispersed in 

 all directions. 



(3) It may pass through the body it meets. 



(4) It may be absorbed by the body. 



